This invention relates to an encoder used by being directly mounted on the drive shaft of a servomotor or the like.
Along with the advances that have recently been made in the field of robots and automatic machines, encoders which serve as devices for detecting rotational positions and positions along a straight line have come into wide use for the purpose of controlling these robots and machines. A great variety of such encoders are available.
Owing to their structural simplicity and low cost, it is most common to use incremental-type encoders to position servo-mechanisms, particularly the rotating portions thereof.
However, for reasons which will now be set forth, there has recently been an increase in demand for absolute-type encoders. Specifically, when controlling the position of a robot, by way of example, the required processing is to sense a home position first and then use this position as a reference when putting the robot through its motions. (With an incremental-type encoder, this operation is required whenever the power supply is turned on.) If this initializing processing for sensing the home position includes an error, all subsequent motions of the robot will include the error. Accordingly, the initializing operation must be performed with great care and can be troublesome. If an absolute-type encoder is used, these settings need be made only once, after which operation can start immediately regardless of how many times the power supply is turned on and off, providing the contents of the particular job do not change. Thus, with an absolute-type encoder, troublesome initializing processing is not required.
There is increasing demand not only for absolute encoders of the conventional type which detect absolute position for a single revolution but also for absolute encoders capable of detecting absolute position for a large number of revolutions.
With the progress that has recently been made in science and technology, devices which utilize automatic control are becoming increasingly smaller in size, and it is required that the encoders used in these devices also be reduced in size.
A rotary encoder is used by being directly mounted on a servomotor shaft. In the case of the so-called "built-in" type encoder, conventionally the encoder used is of the type which will not cause the motor shaft to develop play in the axial direction. In such an encoder, a rotary disk is directly attached to the shaft or the rotary encoder itself is equipped with a plurality of bearings, whereby the rotary disk is securely mounted without allowing motion of the rotary disk other than rotational motion and in such a manner that other structural portions of the rotary encoder are not adversely affected. The entire arrangement is commonly mounted on the motor shaft directly or through use of couplings or the like.
However, a motor shaft generally has some play in the axial direction and disk motion on the encoder side is limited by bearings arranged in two-point support structure even in the built-in type encoder.
An example of this conventional encoder is shown in FIGS. 9(A), (B), in which numeral 20 denotes a rotary disk, 30 a drive shaft for supporting the rotary disk 20, 40 a control base plate, and 81 through 84 support bearings for the drive shaft 30.
This conventional arrangement has the following problems:
(1) The structure is complicated and costs are high.
(2) If a coupling is not used, the motor shaft and the rotary shaft of the encoder may both be damaged unless the axes of the motor shaft and encoder shaft are in alignment.
(3) If the coupling is used, the cost is raised and play in the coupling has an adverse effect, thereby diminishing accuracy and reliability. Another unavoidable problem is that extra mounting space is required in order to accommodate an increase in length due to the coupling.